Apparatus and process for separating air by cryogenic distillation

ABSTRACT

In a process for separating air by cryogenic distillation, an oxygen-rich stream having a first oxygen purity of less than 98 mol % oxygen is withdrawn from the column system and sent to a customer, an oxygen-rich liquid stream having a second oxygen purity, possibly lower than 98 mol % oxygen, is sent to a storage tank, liquid is periodically withdrawn from the storage tank and sent to a back-up reboiler, to produce a back-up product, and a liquid stream having a third oxygen purity, lower than the first purity, is withdrawn from the bottom of at least one column of the column system and/or from the auxiliary reboiler and sent to the storage tank as a purge stream during a stoppage in the operation of the column system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/EP2011/061278, filed Jul. 5, 2011, which claims the benefit of FR1055421, filed Jul. 5, 2010, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an apparatus and to a process for separating air by cryogenic distillation.

BACKGROUND

During shutdown of a distillation apparatus for maintenance or defrosting, it is necessary to evacuate the cryogenic liquids accumulated in the cold box, which are found essentially at the column bottoms and in the bath evaporators.

These liquids cannot be discharged to the atmosphere as they are.

More particularly in the context of an apparatus for cryogenic production of impure (<98%) oxygen intended in particular for oxy-fuel combustion, the proposal is to send all of the liquids present in the cold box to the LOX storage facility, which supplies a standby evaporator.

This also avoids the addition of specific equipment, as described in EP-A-1746374 (which is expensive and takes up a lot of ground space, and which sometimes fails since it is rarely burdened) to manage the purging of the liquids during shutdowns, and also makes it possible to retain the oxygen molecules.

The purges of the cryogenic liquids from the cold box are generally sent to a specific device for their evaporation:

-   -   gravel pit, for generators of very small size;     -   atmospheric chamber, which collects the liquids, which will         subsequently undergo slow evaporation;     -   vaporizer (heating and ventilation).

In EP-A-0605262, a purge liquid is sent to a storage facility, and a liquid portion from the storage facility is sent to a heat exchanger to be mixed with gaseous oxygen. The storage facility is also supplied by liquid transported by a truck: the system is therefore one of injection of liquid oxygen with a contribution of external liquid, thereby allowing the purge to be evaporated with the reheated gaseous oxygen.

WO 2010/017968 describes the sending of an auxiliary evaporator purge to a storage facility.

U.S. Pat. No. 5,566,556 describes a process for separating air wherein the bottom liquid from the low-pressure column is sent to a storage facility as a purge stream.

SUMMARY OF THE INVENTION

It is an aim of the present invention to derive value from the refrigeration potentials of the residual streams and to reduce the investment in the apparatus.

According to one subject of the invention, a process is provided for separating air by cryogenic distillation, wherein:

-   i) all of the compressed and purified air to be separated is cooled     in a heat exchanger, -   ii) cooled air in the heat exchanger is sent to a column system, -   iii) an oxygen-rich stream having a first oxygen purity of less than     98 mol % of oxygen is withdrawn from the column system and sent to a     customer, optionally after evaporation in an auxiliary evaporator     other than the heat exchanger, -   iv) an oxygen-rich liquid stream having a second oxygen purity of     optionally less than 98 mol % of oxygen is sent to a storage     facility from an external source and/or the column system, -   v) liquid is sporadically withdrawn from the storage facility and     sent to a standby evaporator, to produce a standby product in the     event of shutdown of the column system or of increased oxygen     demands from a customer, -   vi) a liquid stream having a third oxygen purity is withdrawn from     the bottom of at least one column of the column system and/or from     the auxiliary evaporator, if present, and it is sent to the storage     facility as a purge stream, characterized in that the liquid stream     is sent to the storage facility during a stoppage of the column     system, and optionally during the restart of the columns, the third     oxygen purity being lower than the first.

According to other, optional aspects:

-   -   the stream having the first purity is withdrawn in liquid form         and is evaporated in the heat exchanger.     -   the liquid stream having the third purity is sent to the storage         facility only if the level of liquid in the storage facility is         above a given threshold.     -   the liquid stream is withdrawn from the auxiliary evaporator.     -   the liquid stream having a third purity is sent to the storage         facility only if no air stream is sent to the column system.     -   the column system comprising a double column consisting of a         medium-pressure column and a low-pressure column, and optionally         an argon column or a column supplied by the medium-pressure         column operating at a pressure intermediate between the medium         pressure and the low pressure, and the liquid having the third         purity comes at least from the bottom of the argon column and/or         the column operating at the intermediate pressure and/or the         low-pressure column and/or the medium-pressure column.     -   the liquid having the third purity comes solely from at least         one column of the system.     -   the liquid having the third purity comes at least from the         auxiliary evaporator.     -   the liquid having the third purity comes solely from the         auxiliary evaporator.

According to another subject of the invention, an apparatus is provided for separating air by cryogenic distillation, comprising a column system comprising a medium-pressure column and a low-pressure column, and optionally an intermediate-pressure column or argon column, a supply line for sending compressed, purified, and cooled air to the column system, a product line for withdrawing an oxygen-rich stream from the low-pressure column and for sending it to a customer, optionally after evaporation in an auxiliary evaporator, a storage facility, a standby evaporator connected to the storage facility, a line for supplying the storage facility with an oxygen-rich liquid, characterized in that it comprises a filling line connected to the storage facility and to the bottom of the medium-pressure column and/or to the bottom of the intermediate-pressure column and/or to the bottom of the argon column.

According to other, optional subjects, the apparatus comprises:

-   -   means for regulating the sending of liquid from a column bottom         of the column system via the filling line to the storage         facility, these means being controlled as a function of the         level of liquid in the storage facility.     -   means for regulating the sending of liquid from a column bottom         of the column system via the filling line to the storage         facility, these means being controlled as a function of the air         flow in the supply line.     -   a filling line connecting the storage facility and the bottom of         the medium-pressure column.     -   a filling line connecting the storage facility and the bottom of         the argon column.

Optionally, the apparatus does not comprise a dedicated evaporator for evaporating a purge stream from the medium-pressure column and/or from the intermediate-pressure column and/or from the argon column, where appropriate.

According to an embodiment of the invention, the purge is able to compensate for the evaporation losses in the storage facility, and the excess is evaporated in the system of the standby evaporation (without recovery of the refrigeration potentials in the cold box).

According to an embodiment of the invention, provision is made to send a purge stream to a liquid oxygen storage facility, optionally by means of a pump, depending on its pressure.

An oxygen-rich stream contains at least 75 mol % of oxygen.

It will be noted that the transfer pump may also be the same as is used for the transfer of the production of liquid oxygen, where appropriate.

The liquid oxygen storage facility is filled with oxygen of “industrial merchant” quality, which is therefore “pure” (99.5 mol %). The sporadic sending of a liquid with a markedly lower average purity will slightly pollute the storage facility, which will remain:

-   -   either at an average level higher than the customer requirement     -   or, temporarily, at an average level lower than the customer         requirement, which will generally be accepted by a customer who         uses oxygen in a combustion process (for example, oxy-fuel         combustion). This lower level will be temporary for two reasons:         -   either the standby evaporation is used (which is more the             general case during shut down of the unit, in order to             continue to supply the customer), and therefore the storage             facility is filled again by means of tanker trucks with             “pure” oxygen         -   or the standby evaporation is not used (the customer is not             consuming), and the evaporation of the liquid in the storage             facility will promote the evacuation of (the lighter)             nitrogen, and will therefore raise its oxygen concentration.

In one embodiment, it is preferred to ensure that the storage facility is not completely filled, so as to allow the addition, when the time comes, of the quantity of purge liquid from the cold box. The quantity of purge liquid remains relatively low in relation to the volume of the storage facility.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 represents an apparatus in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

The invention will be described in more detail with reference to the FIGURE, which illustrates an apparatus according to the invention.

In FIG. 1, a stream of compressed and purified air 1 is cooled in a heat exchanger 3 and divided into three. A stream 7 is withdrawn from the heat exchanger at an intermediate level, expanded in a turbine 9, and sent to a low-pressure column 13 of a double column, in gaseous form. Another stream 5, after having been compressed in the compressor A, is liquefied in the heat exchanger 3, and is expanded in a valve 11 and sent to the medium-pressure column 13. A third stream B is sent directly into the medium-pressure column.

Streams enriched in nitrogen and in oxygen are sent from the medium-pressure column to the low-pressure column in a conventional manner. In the bottom of the low-pressure column 15, liquid oxygen accumulates around the reboiler 17. An oxygen stream 19 containing less than 98% oxygen is withdrawn from the bottom of the low-pressure column 15 and is pressurized by a pump 23 to 2 bar; alternatively, the compression may be hydrostatic. A portion of the oxygen at this pressure is sent to an evaporator 24, in which the portion of the oxygen undergoes evaporation by heat exchange with a portion of the feed air (5 or B), which is then sent to the column. A purge stream 121 containing impurities is withdrawn from the evaporator 24 permanently or occasionally. The evaporated oxygen continues its reheating in the heat exchanger 3 as low-pressure oxygen stream 27.

The remainder of the oxygen is pressurized to a pressure of 10 bar in a pump 25, and is evaporated in the heat exchanger 3 as stream 29. Alternatively, all of the oxygen may be evaporated to the lower pressure, and the stream 29 will not exist.

On shut down of the system (breakdown of main compressor, maintenance or defrosting of the apparatus), a purge stream 21 is withdrawn from the bottom of the low-pressure column 15. This liquid is richer in nitrogen than the stream withdrawn at the column bottom would normally be, since all of the liquids accumulated in the packings in the column fall to the base of the column.

Accordingly, the purge stream is less pure in oxygen than the stream taken in normal operation.

The purge from the bottom of the low-pressure column may also be mixed with a purge from the medium-pressure column withdrawn via line 51, which is even richer in nitrogen than the stream withdrawn at the bottom of the low-pressure column. One or other of these purge streams, or a mixture of the two, are sent to the storage facility 31 if the liquid level measured by the element LL does not exceed a given threshold. If the level of liquid is sufficiently low, the valve 41 opens and the purge liquid is sent to the storage facility.

In one embodiment, when distillation is started up again, the sending of the liquid from the bottom of the low-pressure and/or medium-pressure column is halted.

In another embodiment, it is also possible to send the liquid accumulated in the evaporator 24 via the purge stream 121 from the evaporator to the storage facility 31, alternatively alone, mixed with the purge 21, mixed with the purge 51, or both purges 21 and 51. This liquid is less oxygen-rich than the stream 21 in normal operation.

The majority of the liquid in the storage facility comes from a tanker truck 61 or from the apparatus itself via the stream 21, which can be diverted to the storage facility as and when necessary. The liquid in the storage facility from the truck has a purity of 99.5 mol %.

The pump 23 may optionally serve to send the purge liquid 21 to the storage facility.

In order to supply the customer when the air separation apparatus is not operating or is not producing enough oxygen, a liquid oxygen stream 35 is withdrawn from the storage facility 31 and sent to a standby evaporator 37, which is heated by steam, hot water, air or another fluid.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. A process for separating air by cryogenic distillation, the process comprising the steps of: i) cooling compressed and purified air to be separated in a heat exchanger to form cooled air; ii) introducing the cooled air from the heat exchanger to a column system; iii) sending a first oxygen-rich stream having a first oxygen purity of less than 98 mol % of oxygen from the column system and to a customer; iv) sending a second oxygen-rich liquid stream having a second oxygen purity to a storage facility from an external source and/or the column system; v) sending liquid sporadically from the storage facility to a standby evaporator, to produce a standby product in the event of at least one of the group of shutdown of the column system or of increased oxygen demands from a customer; and vi) sending a third oxygen-rich liquid stream having a third oxygen purity from a source to the storage facility as a purge stream, wherein the source is selected from the group consisting of a bottom portion of at least one column of the column system, an auxiliary evaporator, and combinations thereof, wherein the third oxygen-rich liquid stream is sent to the storage facility during a stoppage of the column system, and wherein the third oxygen purity is lower than the first oxygen purity.
 17. The process as claimed in claim 16, wherein the first oxygen-rich liquid stream is sent in liquid form and is evaporated in the heat exchanger.
 18. The process as claimed in claim 16, wherein the third oxygen-rich liquid stream is sent to the storage facility only if the level of liquid in the storage facility is above a given threshold.
 19. The process as claimed in claim 16, wherein the first oxygen-rich stream is sent to the customer following evaporation in an auxiliary evaporator other than the heat exchanger.
 20. The process as claimed in claim 16, wherein the third oxygen-rich liquid stream is withdrawn from the auxiliary evaporator.
 21. The process as claimed in claim 16, wherein the third oxygen-rich liquid stream is sent to the storage facility only if no air stream is sent to the column system.
 22. The process as claimed in claim 16, wherein the column system comprises a double column having a medium-pressure column and a low-pressure column, and the third oxygen-rich liquid comes at least partly from the low-pressure column and/or the medium-pressure column.
 23. The process as claimed in claim 22, wherein the column system further comprises an argon column and the third oxygen-rich liquid comes at least partly from the bottom of the argon column.
 24. The process as claimed in claim 22, wherein the column system further comprises a column supplied by the medium-pressure column operating at a pressure intermediate the medium-pressure column and the low-pressure column, and the third oxygen-rich liquid comes at least partly from the column operating at the intermediate pressure.
 25. The process as claimed in claim 22, wherein the third oxygen-rich liquid comes solely from at least one column of the system.
 26. The process as claimed in claim 26, wherein the third oxygen-rich liquid comes at least from the auxiliary evaporator.
 27. The process as claimed in claim 26, wherein the third oxygen-rich liquid comes solely from the auxiliary evaporator.
 28. An apparatus for separating air by cryogenic distillation, the apparatus comprising: a column system comprising a plurality of columns, wherein the plurality of columns further comprises a medium-pressure column and a low-pressure column; a supply line for sending compressed, purified, and cooled air to the column system; a product line for withdrawing an oxygen-rich stream from the low-pressure column and for sending the oxygen-rich stream to a customer; a storage facility configured to send and receive an oxygen-rich liquid; and a standby evaporator connected to the storage facility; wherein the apparatus comprises a filling line connected to the storage facility and to the bottom of at least one of the plurality of columns.
 29. The apparatus as claimed in claim 28, comprising means for regulating the sending of liquid from the bottom of at least one of the plurality of columns via the filling line to the storage facility, said means being controlled as a function of the level of liquid in the storage facility.
 30. The apparatus as claimed in claim 28, comprising means for regulating the sending of liquid from the bottom of at least one of the plurality of columns via the filling line to the storage facility, said means being controlled as a function of the air flow in the supply line.
 31. The apparatus as claimed in claim 28, comprising an absence of a dedicated evaporator for evaporating a purge stream from at least one of the plurality of columns.
 32. The apparatus as claimed in claim 28, comprising a filling line connecting the storage facility and the bottom of the medium-pressure column.
 33. The apparatus as claimed in claim 28, further comprising an argon column, and a filling line connecting the storage facility and the bottom of the argon column.
 34. The apparatus as claimed in claim 28, further comprising an intermediate-pressure column.
 35. The apparatus as claimed in claim 28, further comprising an auxiliary evaporator configured to evaporate the oxygen-rich stream from the low-pressure column prior to sending the oxygen-rich stream to the customer. 